The present invention relates to an apparatus and method of evaluating a vascular endothelial function in which the evaluation is enabled with a required minimum configuration by a simple measurement, without using an ultrasonic echo system or the like.
Recently, researches that arteriosclerosis develops while showing deterioration of the vascular endothelial function as the initial phase have been conducted. In order to prevent arteriosclerosis, techniques and apparatuses for evaluating the vascular endothelial function have been developed.
As a reliable technique for evaluating the vascular endothelial function, there is an apparatus called an FMD (Flow-Mediated Dilation) measurement system which is a related art. In the apparatus, measurement is performed in the following manner. A cuff which is similar to that for measuring the blood pressure is attached to the arm of the subject. After avascularization is performed for a constant time of about five minutes at a pressure which is higher than the maximal blood pressure of the subject, the avascularization is released. At about three minutes after the release of the avascularization, the vessel diameter at the upstream or downstream of the cuff is measured by an ultrasonic echo system. Based on the time-dependent change rate of the vessel diameter, the vascular endothelial function is evaluated.
In the case of a normal vessel, the production of NO which is a vasodepressor material from vascular endothelial cells is promoted by shear stress of the inner wall of the vessel due to a blood flow immediately after the avascularization. As a result, the vessel diameter is expanded. By contrast, in the case where a disorder exists in the vascular endothelial function, the degree of the expansion of the vessel diameter is decreased. When the change in vessel diameter before and after the avascularization is measured, therefore, it is possible to evaluate the vascular endothelial function.
The evaluation technique by the FMD measurement system requires skills in measurement of the vessel diameter by an ultrasonic echo system, and is difficult to handle. Furthermore, there is a problem in that the technique requires a large-scale apparatus and lacks in simplicity.
By contrast, as a technique using a simple configuration, there is a related-art technique using a cuff pressure. In the related-art technique, the cuff pressure is maintained at a predetermined pressure which is higher than the maximal blood pressure, thereafter rapidly lowered, maintained at another predetermined pressure which is higher than the minimal blood pressure and lower than the mean blood pressure, and, during when the cuff pressure is maintained at the other predetermined pressure, a ratio of a cuff pressure peak value of a first pulse wave which initially appears to the maximal cuff pressure peak value which thereafter appears is calculated, thereby enabling the vascular endothelial function to be evaluated (refer to JP-A-2007-209492).
As a technique in which an index of the vascular endothelial function can be accurately measured by a simple method, there is a related-art technique in which pressure and volume pulse waves of a vessel to be measured are measured, a ratio of variations of the pulse waves per unit time is obtained, and, with respect to the third root of the maximum value of the ratio of variations of one heartbeat cycle at rest, a ratio to a value after release of avascularization is calculated as the degree of vessel expansion (refer to JP-A-2006-181261).
There is a further related-art technique in which, based on the time-dependent change of posterior pulse wave information indicating a feature of the posterior half part which is after the peak of a pulse wave reflecting variations of the vessel diameter, it is determined whether the function of vascular endothelial cells is normal or not (refer to Japanese Patent No. 3632014).
In the related art disclosed in JP-A-2007-209492, the pressurizing periods for the pressure stimulation and the pulse wave measurement are continuous to each other. Although the pressure for the pulse wave measurement is lower than the artery mean blood pressure, the vein blood flow is blocked, and hence the burden on the subject is large.
In the related art disclosed in JP-A-2006-181261, in addition to the cuff for the pressure stimulation, a sensor for measuring the volume and pressure pulse waves must be disposed. Therefore, the operation is complicated.
In the related art disclosed in Japanese Patent No. 3632014, a reflected wave component which is contained in the pressure pulse wave, and which is originated from peripheral vessels is measured. Measurement of the reflected wave component and calculation of an amplitude augmentation factor AI necessitate complicated waveform recognizing and calculating processes, and an analyzing unit must have a high processing capacity.
The vascular compliance is changed by the blood pressure. When the blood pressure is high, the vessel wall is in a state where the wall is extended in the circumferential direction and hardened, and the compliance is low. Conversely, when the blood pressure is low, a force acting on the vessel wall is small. Therefore, the vessel wall is extended in a smaller degree in the circumferential direction, and the compliance is high. All of the related arts disclosed in JP-A-2007-209492, JP-A-2006-181261, and Japanese Patent No. 3632014 have a problem in that the measured vessel information is inevitably affected by the intravascular pressure, i.e., the blood pressure.